In an automotive vehicle, electromechanical hydraulic valves (incorporating a solenoid coil) are typically employed in an anti-lock braking system of the vehicle. Such valves may be actuated by electrical control circuits comprising semiconductor components such as power MOSFETs (metal oxide silicon field effect transistor). The valves, and consequently the hydraulic pressure in a braking system, can be controlled using a pulse width modulated driver stage incorporating at least one power MOSFET.
One known valve control circuit is shown in FIG. 1 and indicated generally at 100. A similar arrangement is described in US-A1-20090299592. In FIG. 1 an inductive load 101 (for example the coil of an electromechanical hydraulic valve) is controlled by a high side driver circuit 102 and a low side driver circuit 103. The high side driver circuit 102 comprises a high side driver 104 and a high side transistor 105. An output of the high side driver 104 is connected to the gate of the high side transistor 105. The low side driver circuit 103 comprises a low side driver 106 and a low side transistor 107. An output of the low side driver 106 is connected to the gate of the second transistor 107. The high side and low side transistors 105, 107 act as switches and may comprise MOSFET devices. The coil 101 is connected across the high side transistor 105 and between a voltage source V and the low side transistor 107. In the example of FIG. 1, the high side driver 104 controls the gate of the high side transistor 106 in a PWM (pulse width modulated) mode according to a PWM control signal which is applied to an input of the high side driver 104. An additional low side driver PWM control signal applied to the input of the low side driver 106 controls the gate of the second low side transistor 107. The high side and low side transistors 105. 107 are driven in opposite phase. When the low side transistor 107 is on, current flows through the coil 101 via an actuation path 108 and the current in the coil rises exponentially. When the low side transistor 107 is turned off, the high side transistor is switched on so that a continuous flow of the coil current is possible via a recirculation path 109.
FIGS. 2A, 2B, 2C illustrate the current variation through the coil 101 (FIG. 2A) and the PWM control signals applied to the low side transistor 107 and high side transistor 105 (FIG. 2B, FIG. 2C respectively). During the time period ton during which low side transistor 107 is on (and transistor 105 is off) the coil current rises towards a maximum value. During the subsequent time period toff during which the low side transistor 107 is off (and transistor 105 is on), the coil current decreases. The process then repeats under the control of the PWM signals generated by each driver 104, 106. Thus an average current value in the coil 101 may be set by adjusting the duty cycle of the PWM signals. A current value may be regulated by comparison with a defined target current. Certain automotive systems such as adaptive cruise control, for example, require very precise valve current control even in very small current ranges (of around 100 mA typically). Such currents correspond to very low PWM duty cycles.
As an automotive braking system is a safety-critical system, a failsafe operation of the valve control circuit is desirable. Therefore, not only is it necessary that the valve current be precisely regulated but that any defect in the valve control circuit should be detectable so that appropriate action may be taken. Detection of a defect in the control circuit may result in disconnection of the associated control unit or switching over to an auxiliary/emergency system. It is usual practice to incorporate some redundancy in safety critical functions. In the particular case of electromagnetic valve actuation for automotive braking system control, the valve coil current may be measured both in the actuation path (i.e. low side current) and the recirculation path (i.e. high side current). A comparison may be made between the two measurements whereby detection of any discrepancy indicates a failure somewhere in the control circuit
U.S. Pat. No. 5,763,963 describes a circuit arrangement for controlling the current in inductive loads by pulse width modulation. For safety purposes, a second redundant current measuring is performed by way of an additional switching transistor in parallel with the low side driver switch. This additional transistor is actuated at preset time intervals. However, this arrangement does not guarantee redundancy during each PWM period. Furthermore the diagnostic current measurement is performed by the additional parallel circuit branch rather than by the existing current regulation loop and the additional circuitry is area-consuming as the additional low side transistor size may be the same as the low side power transistor.
U.S. Pat. No. 7,047,119 discloses a circuit arrangement for controlling current through an inductive load by pulse width modulation wherein the current measurement takes place as close as possible to ton/2 in the actuation path or toff/2 in the recirculation path (see FIGS. 2B and 2C). Such measurements may give a reasonable estimate of the mean value of the coil current. An analogue-to-digital converter (ADC) is employed in circuitry for measuring the current. However, this arrangement has a lower limit (ton_min) with respect to its shorter adjustable duty cycle of ton_min=2Tconv, where Tconv is the conversion time of the ADC. So below this lower limit (that is; for very low duty cycles), there is not enough time to measure current in the actuation path so only the current in the recirculation path can be used for valve current regulation. Therefore current measurement redundancy is lost. The same analysis applies for a lower limit for toff at high duty cycles.
US 20090299592 A1 discloses an electronic controller that uses pulse width modulation to control inductive load currents and incorporates an ADC which is a sigma-delta modulator. For very small duty cycles the power device in the actuation path is switched on at defined times for a short time to allow current measurement in the actuation path. Similarly, for very high duty cycles the power device in the actuation path is switched off for a short time to allow current measurement in the recirculation path. This arrangement has the disadvantage of causing some noise in the current regulation system.